To realize the weight saving of an automobile, application of high-strength steel sheets to automobile parts has been expanded in recent years. Since the degree of an elastic recovery (hereinafter referred to as a springback, or may be abbreviated as SB) of high strength steel sheets after a press forming is greater than that of mild steel sheets, it is difficult to ensure accuracy of dimensions of parts. Therefore, until an acceptable dimensional accuracy is achieved, a repeated correction of a shape of press die and springback compensation of die are required.
To reduce the burden for such correction of a press die, techniques for estimating the amount of springback at the press die design stage have been demanded, and analysis systems for the estimation have been developed.
The analysis method involves the following stages:
(1) analysis of the deformation, stress, and strain of a part under the restraint by a press die; and
(2) analysis of the springback after the release from the restraint by the press die.
Note that the analysis in stage (1) is performed by using a finite element method, and the analysis in stage (2) is performed by using a springback theoretical formula or a finite element method.
The springback is known to be caused by an uneven distribution of residual stress accumulated in a formed part before the formed part is released from a press die. Conventionally, estimation using a numerical analysis method, such as a finite element method, has been performed. However, it has been difficult to estimate which region of the formed part has residual stress having a dominant influence on the springback.
For example, Patent Literatures 1 and 2 discuss influences of the shape and the forming conditions of a formed part on the springback.
Patent Literature 1 discloses a method of calculating how a value, which is defined to describe the springback, changes when a distribution of residual stress in a specific region of a press-formed part changes, and estimating an influence of the distribution of residual stress in the specific region on the springback.